The mare I ride (the one from an other answer i did you) has kissing spine and she cannot canter because vertebrae rubs much more. That may be one of the reasons of no canter.
Hope your question was answered.
she canters and gallops really well in the field. And she used to be really good at cantering when I just sat into the saddle and she’d go, now she sort of throws her head and trots really fast, even on lunge she just trots really fast. But I will try what you have said thank you :))
First of all you need to see a vet to be sure there is no physical problem. Maybe he is missing some muscles to support the canter because of the long stop.
Is he cantering in the field?
Then, if there is nothing, you need to be sure that you are respected by keeping the action until he gives in and don't forget to reward him once he does it.
Try to go from the walk so he will be calmer.
When you lunge him, be aware of your position towards him (always on his hindquarters to give impulsion and stimulate with the lunging whip if it's not enough), you can also try to give him more space in lunge by running beside him (you will also motivate him to keep doing the effort).
You can also try to ask your request behind a horse that is already cantering, it will help him to go, or outside the arena, he will be more motivated to canter.
Did he canter before or is it just since the beak?
I hope i could help you :)